1. Field of the Invention
The present invention relates to a magnetic head installed drum for use in a magnetic recording/reproducing device such as a video cassette recorder (VCR), and also to a method for adjusting the position of a rotary head mounted in the magnetic recording/reproducing device.
2. Description of the Prior Art
As shown in FIGS. 1 and 2, high quality consumer VCR decks today have plural rotary heads 1 provided on a rotary drum 3 for recording video and hi-fi audio signals onto the tape, and for erasing recorded tracks. Each of the rotary heads 1 is attached to a metal base called the head base 2, and the head bases 2 are mounted on the rotary drum 3. Each of the rotary heads 1 must be precisely positioned on the rotary drum 3 in order to maintain the integrity of the tape format because the position of the tracks recorded to the tape is primarily determined by the position of the heads on the rotary drum 3.
The position of the heads is defined by the absolute head height Ha, the relative head height Hr, and the azimuth angle 7 of the head gap. The absolute head height Ha is the distance from the reference surface 6 of the fixed drum 4 and the track edge 5 of the head 1, and the relative head height Hr is the distance between the track edge of the different heads.
The head height has conventionally been adjusted as follows.
Consumer VCR decks do not typically have a mechanism for adjusting the azimuth angle 7, and the azimuth angle 7 is therefore adjusted when the heads are manufactured by cutting a gapped bar into chips. The precision of the azimuth angle 7 is therefore determined by the mechanical precision of the cutting process, and by the precision of the process by which the head is attached to the head base 2 with an adhesive 8.
To adjust the head height, the distance H0 is first measured for each head from the surface of the head base 2 to which the rotary heads 1 are attached to the track edge 5 of the heads. The absolute height of each head is then roughly adjusted for the distance from the head base 2 surface to the track edge 5 of the head by inserting metal plates 9 called "shims" according to the amount of adjustment required. Each of the heads is then secured to the rotary drum 3 with a screw 10.
The rotary drum 3 is mounted to the fixed drum 4 after the heads are mounted to the rotary drum 3, and the absolute height and relative height of each head is then precisely adjusted using the screw 11 contacting the head base 2 as shown in FIG. 2.
Elastic deformation of the head base 2 caused by loosening or tightening the screw 11 causes the rotary head 1 mounted on the head base 2 to move vertically in head height direction B (parallel to the shaft 12 of the rotary drum 3), and makes it possible to precisely control both the absolute head height Ha and the relative head height Hr. In practice, however, differences in the shape of the screw 11 tip and the translation of one full turn of the screw 11 into a 500 .mu.m change in the head height makes it very difficult to achieve submicron adjustments in the head height.
Furthermore, the simple adjustment described above depends upon the mechanical deformation of the head base 2 caused by the pressure of the screw 11 against and within the limits of the elastic deformation of the head base 2. As a result, when the drum unit is subject to mechanical vibrations and changes in the ambient environment, the screw 11 pressing against the head base 2 tends to be loosened, resulting in a change in the head height.
Studies conducted by the inventors have shown that the relative head height can change approximately two microns in vibration tests and in heat cycle tests in which the temperature is varied between -40.degree. C. and +80.degree. C.
A change in the head height of such magnitude results in a significant deterioration in the quality of the playback image, particularly during the extended play mode, which in the VHS format is a narrow track six-hour playback mode. Technically, a change in the head height of this degree is a cause of tracking error, reduced playback voltage, and increased crosstalk from adjacent tracks.
In addition, the rotary head 1 to tape angle can be greatly increased if the screw 11 depressing the head base 2 is screwed in too far as shown in FIG. 3. This results in poor contact between the tape and head during both recording and playback and lowers the playback output level. As a result, the effective adjustment range using the screw 11 has been five microns.